1. Field of the Invention
The invention relates to an intake port structure for an internal combustion engine. More particularly, the invention relates to an intake port structure for an internal combustion engine, which avoids the locking of an airflow control valve, which increases the strength of a swirling airflow generated in a combustion chamber, and which suppresses the deterioration of fuel efficiency and reduces emissions.
2. Description of the Related Art
As a technology related to the invention, an intake port structure for an internal combustion engine is known. In the intake port structure, an airflow control valve provided in an intake port is controlled to be opened/closed to control the flow of intake air, and to generate a swirling flow such as a tumble flow and a swirl in a combustion chamber. The swirling airflow, such as the tumble flow and the swirl, promotes the mixing of fuel and air, and the transmission of flame to improve combustion efficiency. Also, the swirling airflow brings rich air-fuel mixture near to an ignition plug to enable stratified charge combustion. Japanese Utility Model Application Publication No. 7-25264 (Patent Publication 1) describes an airflow control valve (shutter valve) used to generate the swirling airflow.
In the technology described in Patent Publication 1, the airflow control valve is provided on the wall surface of the passage of an intake port. In the intake port, a first passage and a second passage are formed by providing a partition wall. Intake air is guided to the center portion of the space inside a combustion chamber through the first passage so that the tumble flow is generated. The intake air is guided to the outer peripheral portion of the space inside the combustion chamber through the second passage. One end of the airflow control valve is supported by a shaft so that the airflow control valve can pivot. When the airflow control valve is partly opened, the other end of the airflow control valve contacts the end portion of the partition wall. As a result, the entire cross sectional area of the second passage is closed. When the airflow control valve closes the entire cross sectional area of the second passage, the intake air flows into the combustion chamber through the first passage, and the tumble flow is generated. By partly opening the airflow control valve, the strong tumble flow is generated in the combustion chamber. By completely closing or completely opening the airflow control valve, the flow amount of intake air can be controlled.
In the intake port structure that includes the airflow control valve whose one end is supported by the valve shaft, the intake airflow moves in the intake port as described below. FIG. 9A to FIG. 9C show an intake port structure 200 that includes an airflow control valve 201 whose one end is supported by a valve shaft 202. The valve shaft 202 is provided at a position near the wall surface 203ab of the intake port 203 in the side view of the intake port 203. More specifically, FIG. 9A shows the distribution of the flow speed of an intake airflow F in the intake port 203 when intake air flows into the intake port structure 200. FIG. 9B is a sectional view of the intake port 203 shown in FIG. 9A, taken along line B-B. FIG. 9C is a sectional view of the intake port 203 shown in FIG. 9A, taken along line A-A.
When the intake air flows into the intake port structure 200 shown in FIG. 9A, immediately after the intake airflow F passes through a passage E that has been made narrow, the intake airflow F is deflected toward a wall surface 203aa, and accordingly, the flow speed of the intake air is increased. However, as the deflected intake airflow F approaches a combustion chamber 204, the intake airflow F is gradually diffused from the wall surface 203aa toward a wall surface 203ab. Accordingly, the flow speed of the intake air is decreased. This reduces the strength of the swirling airflow generated in the combustion chamber 204 based on the intake airflow F.
Japanese Patent Application Publication No. JP-A-6-248956 (Latent Publication 2) describes an engine intake apparatus to solve the above-described problem. The engine intake apparatus is applied to an internal combustion engine in which an intake airflow (air-fuel mixture airflow) is guided to the outer bent portion of an intake port so that a strong tumble flow can be generated in a combustion chamber. The engine intake apparatus includes a sub-port. One end of the sub-port is connected to the inner bent portion of the intake port at a position immediately upstream of an intake valve so as to inject assist air in the direction opposite to the direction of the intake airflow passing through the intake port. The other end of the sub-port is connected to an intake passage at a position upstream of a throttle valve. Air is sucked into the sub-port from the intake passage at the position upstream of the throttle valve due to the difference between the pressure in the intake port and the pressure in the intake passage at the position upstream of the throttle valve. Further, the sucked air serves as the assist air, and deflects the intake airflow toward the outer bent portion of the intake port. In this technology, the stronger tumble flow can be generated in the combustion chamber using the deflected intake airflow.
Japanese Utility Model Application Publication No. 7-42407 (Patent Publication 3) describes an engine intake apparatus. Like the engine intake apparatus described in Patent Publication 2, this engine intake apparatus is also applied to an internal combustion engine in which an intake airflow (air-fuel mixture airflow) is deflected to the outer bent portion of an intake port so that a strong tumble flow can be generated in the combustion chamber. Also, the engine intake apparatus includes a tumble port (sub-port). One end of the tumble port is connected to the inner bent portion of the intake port at a position upstream of an intake valve. The other end of the tumble port is connected to an intake passage at a position upstream of a throttle valve. Further, the tumble port of the engine intake apparatus is formed such that the inner bent portion of the tumble port is inclined at 135 degrees with respect to the direction in which the intake air flows. In this technology, the intake airflow is outwardly deflected by assist air injected from the tumble port. Thus, the stronger tumble flow can be generated in the combustion chamber.
When the airflow control valve described in Patent Publication 1 is partly opened, an engulfing flow is generated in the intake port, and is directed toward the space between the lower surface of the airflow control valve (i.e., the lower surface in the direction of the intake airflow) and the wall surface of the intake port. The intake airflow contains, for example, a small amount of oil and carbon, and powder dust. Oil and carbon flow from a PCV (Positive Crankcase Ventilation System), an intake manifold, and the like that are provided upstream of the intake port. The powder dust is contained in atmospheric air. Therefore, the oil adheres to the lower surface of the airflow control valve and the wall surface of the intake port due to the engulfing airflow directed toward the above-described space. Also, the carbon and the powder dust are deposited, and a deposit is generated. The pivot movement of the airflow control valve is adversely affected by the deposit thus generated. Further, the airflow control valve may be locked due to the deposit.
In the intake port structure 200 shown in FIG. 9A, the intake airflow F is contracted in the area between the stems 205a of an intake valve 205, as shown in FIG. 9B that is a sectional view of the intake port 203 taken along line B-B in FIG. 9A. Further, as shown in FIG. 9C that is a sectional view of the intake port 203 taken along line A-A in FIG. 9A, the intake airflow F passes through the area between the stems 205a of an intake valve 205, and the intake airflow F hardly passes through an X portion. In such a situation, if the fuel adheres to the X portion of the intake valve 205, the fuel keeps adhering to the X portion, because the intake airflow P hardly passes through the X portion. The fuel efficiency of the internal combustion engine deteriorates, and the air-fuel ratio of the air-fuel mixture changes due to the fuel adhering to the X portion. As a result, emissions contained in exhaust gas, such as CO and HC, may increase. However, in Patent Publication 1, no consideration is given to the problem that the airflow control valve may be locked, and the problem that the fuel efficiency of the internal combustion engine may deteriorate, and the emissions may increase due to the oil adhering to the airflow control valve.
The engine intake apparatuses described in Patent Publications 2 and 3 can increase the strength of the generated tumble flow by deflecting the intake airflow, as compared to the airflow control valve described in Patent Publication 1. However, because the assist air is injected in the direction opposite to the direction of the intake airflow, the intake airflow may be attenuated, and accordingly, the strength of the generated tumble flow may be reduced.